In the following, the function of a traditional coffee machine will be described. Note that the term “coffee machine” also includes “coffee brewer” and “coffee maker” in the following discussion.
A traditional coffee maker normally works by admitting water from a cold water reservoir into a flexible hose in the base of the reservoir leading directly to a thin metal tube or heating chamber (usually, of aluminum), where a heating element surrounding the metal tube heats the water. The heated water moves through the machine using the thermo siphon principle. Thermally-induced pressure and the siphoning effect move the heated water through an insulated rubber or vinyl riser hose, into a spray head, and onto the ground coffee, which is contained in a brew basket mounted below the spray head. The coffee passes through a filter and drips down into the carafe. A one-way valve in the tubing prevents water from siphoning back into the reservoir. A thermostat attached to the heating element turns off the heating element as needed to prevent overheating the water in the metal tube (overheating would produce only steam in the supply hose), then turns back on when the water cools below a certain threshold.
It is crucial for the quality of the coffee that the water temperature as it hits the ground coffee is properly adjusted. Too low or too high a temperature will degrade the taste of coffee. Ninety-two degrees Celsius is often referred to as the ideal outlet temperature. The problem with prior art coffee makers is that even if they control of the temperature as it leaves the heater, there is no actual control of the temperature as the water hits the ground coffee. This is partly because traditional coffee makers use steam as a driving force for water movement between the water tank and filter holder. Further, coffee makers are not adapted to changes in the environment related to pressure, humidity and temperature, in addition to being poorly protected against frost damage that may occur in the piping in and around the heating element. Frost damage typically occurs if a machine is used in a house that is not insulated winter, and when the coffee is maker not emptied of water.
It is therefore desired to provide a coffee machine that allows for control of the temperature of the water as it hits the ground coffee and that is adapted for changes in the environment.